The IEEE 802.11n standard defines a protocol enabling data transmission on a 40 MHz bandwidth in grouping together or “concatenating” two adjacent 20 MHz channels (the WiFi classically works on 20 MHz channels).
A protocol of this kind defines a primary channel to which a channel-assigning mechanism, known as the CSMA-CA (carrier sense multiple access with collision avoidance) mechanism, is applied, and a secondary channel, on which the CSMA-CA mechanism is not performed. This secondary channel however is scanned by means of a physical layer mechanism called a CCA (Clear Channel Assessment) in order to check the occupancy of this channel. This check is made for a duration denoted as PIFS (or “point coordination function interframe spacing” of 25 to 36 μs according to the IEEE 802.11n standard), when transmission on a 40 MHz frequency bandwidth is envisaged.
For reasons of backward compatibility, the transmission of payload data on a channel with a 40 MHz bandwidth, called a “concatenated” channel, must be preceded by the transmission of preambles on the primary channel (of 20 MHz bandwidth) and on the secondary channel (also of 20 MHz bandwidth), and must be understood by any station that can receive data only on a 20 MHz bandwidth.
However, the receiver stations do not know if the forthcoming transmission will be made on a 20 MHz bandwidth or a 40 MHz bandwidth. They must therefore receive these preambles on a 20 MHz bandwidth only, preferably on the primary channel, because the secondary channel may be occupied by transmissions from other stations or other access points (AP), and they must receive these preambles until reception of an HT-SIG2 signaling field, where a signaling bit informs the receiver of the bandwidth used for the transmission in progress (20 MHz or 40 MHz).
In the case of a transmission using a 40 MHz band, the receiver must then change its bandwidth in order to receive data on a 40 MHz bandwidth (corresponding to a primary channel and to the secondary channel) and no longer on a 20 MHz bandwidth (corresponding to the primary channel).
Present-day receivers generally adopt permanent reception on a 40 MHz bandwidth. However, such a technique has the drawback of incurring interference on the secondary channel during a transmission limited to 20 MHz on the primary channel alone. Indeed, the secondary channel can be occupied by an adjacent station or access point and the data transmitted on this secondary channel must also be received by the receiver. In particular, the drawback of this approach lies in the fact that the ratio between the amplitudes of the data to be transmitted on these two channels is not controlled. Indeed, if the secondary channel carries data having an amplitude greater than the amplitude of the data of the primary channel by several tens of decibels (dB), then the reception on the primary channel can be greatly deteriorated, or even be completely compromised.
To overcome these drawbacks, two prior-art solutions have been proposed, respectively implementing reception with dual frequency change, corresponding to what is known as the superheterodyne principle illustrated in FIGS. 1A and 1B, or reception with direct conversion, also known as “Zero-IF” reception illustrated in FIGS. 2A and 2B (the sense of the reception being illustrated by the arrow F).
The dual frequency change receiver is shown in FIG. 1A in a configuration for receiving a signal SR transmitted in a 20 MHz pass-band channel. The received signal is a radiofrequency signal, in a frequency bandwidth situated between 5 and 5.9 GHz for example, filtered with this bandwidth by a first filter 11, and then converted into an intermediate frequency signal. Then, a second selective filter 12 of 20 MHz width eliminates the potentially disturbing adjacent signals.
As illustrated in FIG. 1B, to pass from transmission on a 20 MHz bandwidth to transmission on a 40 MHz bandwidth with the aim, in each case, of maintaining the best possible quality of reception, the dual frequency change receiver must change its intermediate frequency (with a 10 MHz offset) and its filter to adapt to the broadening of the bandwidth. In this case, the 20 MHz bandwidth filter is replaced by a 40 MHz bandwidth filter.
The second principle of reception, of a Zero-IF type, is shown in FIG. 2A. In this configuration, the signal is transposed directly into baseband by an oscillator whose frequency is the center frequency of the channel sought. The elimination of the potentially disturbing adjacent signals is ensured this time in baseband by highly selective low-pass filters.
As illustrated in FIG. 2B, to pass from 20 MHz transmission to 40 MHz transmission with the aim, in each case, of maintaining the best possible quality of reception, it is the radiofrequency transposition oscillator of the Zero-IF type receiver that must be offset in frequency and the baseband low-pass filters that must be broadened from 10 MHz to 20 MHz.
Thus, in these two techniques of reception, the dynamic passage, during transmission, from implementation at 20 MHz to implementation at 40 MHz of channel width comes up against:                changes of the intermediate frequency (IF) or baseband filters that will necessitate a certain time of adaptation in amplitude of the data to be transmitted and in terms of delay in particular, these changes therefore possibly causing an interruption in transmission (through loss of synchronization),        change in the frequency of the oscillators in intermediate frequency (IF) or RF responsible for making frequency transpositions, which could causes the same disturbances as in the case of the switching of the filters.        
Thus, the inventors have noted that, at present, the reconfiguration of the receiver from a 20 MHz reception bandwidth to a 40 MHz reception bandwidth is a blocking point because no simple and satisfactory method can be directly seen, even partially, for managing a dynamic modification of bandwidth from 20 to 40 MHz without causing disturbance on a transmission preliminarily set on a 20 MHz bandwidth.
Here below, the term “configuration of reception” refers to a mode of reception characterized by its reception bandwidth. For example, in the case applied to the IEEE 802.11n standard, there will be a first 20 MHz configuration of reception corresponding to reception on a 20 MHz bandwidth, and a second 40 MHz configuration of reception corresponding to reception on a 40 MHz bandwidth.